The present invention deals with the data transmission in full duplex on a two-wire line, and relates particularly to a device for cancelling echoes of the outgoing data signals due to the connections between the two-wire and four-wire line portions.
A family of modems is for use on connections between data terminal equipments by means of the switched telephone network. The data signals are transmitted from a transmitting modem to a receiving modem over a two-wire line. For long distances, amplifiers (repeaters) are required. Since amplifiers only operate in one direction, the data directions are split up to separate the two-wire line into a four-wire line. The connection between two and four-wire lines, and vice-versa, is made through so called hybrid couplers. Likewise, an hybrid coupler is required to connect the modem output comprising the transmitting circuit and the receiving circuit, to the two-wire line.
An hybrid coupler is a terminating set consisting of two differential transformers with three inputs, an input for each two-wire circuit of the four-wire line, and an input for the two-wire line. Because these transformers cannot be loaded with an impedance matched throughout the overall frequency bandwidth, a portion of the data signal energy goes over to the other direction and returns to the sender through the two-wire line, generating so called echoes.
Two kinds of echoes are received by the modem: the near echo resulting from the transmitted data signal leaking directly through the modem hybrid coupler toward the receiving circuit of the modem, and the far echo resulting from the transmitted data signal passing through the four-wire line and reflecting on a far hybrid coupler.
Therefore, the modems of this type are provided with echo cancellers to cancel from the incoming signal echoes of the outgoing signal. Because near and far echoes have different characteristics, it is classical to have the near echo canceller different from the far echo canceller, the outputs of both echo cancellers being added to provide an estimated value of the actual echo. This resulting estimated value is then subtracted from the incoming signal to provide theoretically the incoming signal free of echo. The error signal between the estimated and actual values of the echo is generally used to adjust the coefficients of the echo cancellers.
In fact these echo cancellers of the prior technique present drawbacks which are overcome by the present invention.
Indeed, the data signal processing in the present day modems is performed digitally under the control of a processor incorporated in the modem. Accordingly, all the operations of echo cancelling are carried out digitally resulting in an estimated echo value in digital form. This estimated value is then converted by a digital-to-analog converter into analog form before being subtracted from the incoming signal. The resulting signal has, in turn to be converted by an analog-to-digital converter into digital form so as to be used as a control signal for adjusting the echo canceller. As the incoming signal can have a wide energy level range, it is necessary to have a programmable gain amplifier (PGA) at the input of the analog-to-digital converter. The PGA value is set at the beginning of the transmission, by measuring the signal energy at the A/D converter output signal. It may happen that, because of a noise burst for instance, this energy measurement might not be accurate, especially if fast set-up is required, resulting in a bad adjustment of the PGA. Thus, observing a signal with low energy at the A/D converter output may result from either a good cancellation of the echo or from a bad adjustment of the PGA, without the possibility of distinguishing between the two possibilities.
Another drawback of the prior echo canceller is that the digital-to-analog converter which converts the estimated value of the echo into analog form, introduces a frequency distortion of the type Sin X/X. Therefore the control signal used for adjusting the echo canceller coefficients is a filtered version of the true signal, which leads to an imperfect echo cancellation, especially when the sampling frequency is low.